1. Field of the Invention
The present invention relates to a focus detection apparatus and method for detecting the focus state of an imaging lens by using an image generated from beams having passed through different pupil regions of the imaging lens, and an image capturing apparatus.
2. Description of the Related Art
Conventionally, focus detection methods used in digital cameras serving as image capturing apparatuses include a contrast detection method popularly used in video cameras and compact cameras, and a phase difference detection method popularly used in single-lens reflex cameras. In general, a phase difference focus detection method used in single-lens reflex cameras is used to capture a still image because detection is performed by splitting a beam into those for a viewfinder optical system and focus detection optical system by a half mirror or the like inserted in the light path. The focus detection optical system is normally formed from two or more reimaging lenses which receive beams having passed through different pupil regions of the imaging lens, a stop, and an image sensor.
In the phase difference focus detection method, the focus state of the imaging lens is calculated based on an image shift amount between two focus detection images generated from beams having passed through different pupil regions of the imaging lens. The image shift amount between two focus detection images is calculated by performing correlation calculation for the two focus detection images. This calculation method is disclosed in, for example, Japanese Patent Laid-Open No. 63-18314.
When a moving image is viewed or recorded in a camera which captures an image by an image sensor, a contrast detection focus control method is used generally. In the contrast detection focus control method, focus control is performed by extracting the high-frequency component of an image sensed by the image sensor while moving the focus lens of the imaging lens along the optical path, and detecting a lens position where the contrast peaks. However, the contrast detection focus control method cannot perform focus control quickly because the contrast levels of images are compared while moving the focus lens. In recent years, even a single-lens reflex camera can capture a moving image, and it is desired to perform focus control quickly even during moving image capturing.
To meet this demand, the present applicant discloses, in Japanese Patent Laid-Open No. 2001-124984, an image capturing apparatus which can perform phase difference focus detection by arranging two photoelectric converters for one on-chip microlens forming part of a CMOS image sensor. In the image sensor disclosed in Japanese Patent Laid-Open No. 2001-124984, the on-chip microlens is configured so that the pupil of an imaging lens and two photoelectric converters have a conjugated relation. As a result, beams having passed through different pupil regions of the imaging lens are incident on the respective photoelectric converters. By using images obtained by the respective photoelectric converters, known phase difference focus detection can be performed.
Japanese Patent Publication No. 62-37364 discloses a focus detection method of, when detecting the focus state of an imaging lens by using a focus detection image generated from beams having passed through different pupil regions of the imaging lens, obtaining the focus state by detecting the phase of a predetermined spatial frequency of the focus detection image.
However, in the method disclosed in Japanese Patent Publication No. 62-37364, when the focus state is obtained by using the phase of a predetermined spatial frequency of a detected focus detection image, no high-accuracy focus detection can be performed unless the object contains the predetermined spatial frequency component used for detection.